Technical Field
The present invention relates to a pattern critical dimension measurement equipment and a pattern critical dimension measurement method using the same.
Background Art
A scanning electron microscope (SEM) is a type of charged particle beam equipment having a charged particle detection system, and forms an image by detecting signals, such as secondary electrons, generated from a sample when the sample is scanned with an electron beam. SEM is used for various applications. For example, SEM is used to measure the size of a fine circuit pattern, such as a semiconductor device, on a substrate. In addition, equipment that measures the size of a circuit pattern of a semiconductor device using SEM is referred to as a critical dimension SEM, for example.
Nowadays, the sizes of circuit patterns fabricated in a production line of semiconductor devices are measured in each process to improve the production yield of the semiconductor devices. Conventionally, optical microscope-based measurement has often been used for this type of measurement. However, nowadays, in which circuit patterns have been miniaturized, a pattern critical dimension measurement equipment typified by a critical dimension SEM is often used.
However, as the miniaturization of circuit patterns has further advanced, it has become even difficult to discriminate between projections and recesses of a pattern from a SEM image in some cases. FIG. 1 shows a typical example thereof. FIG. 1 shows a SEM image 102 that is obtained by imaging a wafer, which has projections/recesses 101 formed thereon, with a conventional pattern critical dimension measurement equipment, and a line profile 103 thereof. The line profile 103 is a graph obtained by, with respect to each point of the SEM image 102 in the x direction, accumulating the brightness values of the image in the y direction. The pattern critical dimension measurement equipment measures the size of the pattern formed on the wafer on the basis of the line profile 103.
However, the line profile 103 shows no clear difference in brightness between portions corresponding to the projections and portions corresponding to the recesses of the projections/recesses 101, and also shows no difference in brightness between the right and left sides of each pattern edge. Thus, depending on the line profile 103, it would be difficult to discriminate which portion of the SEM image 102 is a projection or a recess.
In order to allow the projections and recesses of the pattern formed on the wafer to be detected from the line profile 103, it would be important to obtain a SEM image with a shadow (i.e., shadow image) equivalent to a shadow that is generated when the projections/recesses 101 are irradiated with light from obliquely above.
FIGS. 2A to 2C illustrate the basic principle to obtain a shadow image. Herein, as shown in FIG. 2A, a case will be described where the surface of a wafer is scanned with a primary electron beam 201. In FIG. 2A, a scanning route 202 of the primary electron beam 201 is indicated by an arrow. Upon irradiation with the primary electron beam 201, secondary electrons A 203 are emitted from the projections/recesses 101. At this time, due to the edge effect, the number of secondary electrons A 203 that are emitted in the left direction is increased on the left side of each projection, while the number of secondary electrons A 203 that are emitted in the right direction is increased on the right side of each projection.
Therefore, the detection counts of secondary electrons A emitted from the projections that are detected by a left detector 204 and a right detector 205 are different. FIGS. 2B and 2C show SEM images obtained with the left detector 204 and the right detector 205, respectively. In FIGS. 2B and 2C, the shadows of the SEM images are enhanced. As shown in FIGS. 2B and 2C, the two SEM images have an opposite relationship of shadows corresponding to projections due to the difference in the detection count of the secondary electrons A 203. As described above, the conventional critical dimension SEM equipment obtains shadow images, which reflect the projections/recesses 101 on the wafer, using the detectors 204 and 205 that are arranged on the right and left sides of the primary electron beam 201.
Patent Document 1, for example, discloses an equipment configuration for obtaining shadow images on the electron source side than on the objective lens side. In Patent Document 1, shadow contrast is obtained by causing secondary electrons, which have been generated from a sample and have passed through an objective lens, to impinge upon a reflector, and detecting secondary electrons generated from the reflector with detectors that are arranged on the right and left sides of the reflector. As described above, using a plurality of detectors can discriminate projections and recesses of a pattern even when an observation object is unknown.